The present invention relates to a testing method of an electrochemical gas sensor used for measuring the component concentration of a carbon monoxide gas. In more detail, the present invention relates to a testing method for judging an operation condition of an electrochemical gas sensor for use in a gas alarm unit which is ordinarily provided in a ship or manhole, tunnel or home, and widely used for the prevention of poisoning accidents, caused by the blowout gas or the exhaust gas of a heater or car, the early detection of the fire, the prevention of fire caused by explosion, and the like.
The electrochemical gas sensor is a sensor which introduces the gas component to be detected onto a working electrode having a catalytic action through a membrane, and outputs the voltage or current corresponding to the gas concentration by oxidizing or reducing the gas, and because the sensor has a small size and light weight, and operates under the normal temperature and normal pressure, and is highly reliable and relatively low cost, it is widely used for the poisoning alarm unit or the industrial measuring unit, and the like.
Conventionally, a general structure of the widely and practically used electrochemical gas sensor is as shown in FIG. 1. Further, a general electric circuit which electrically drives such the electrochemical gas sensor, and by which the output is obtained, is shown in FIG. 2. According to FIG. 2, the principle of operation of the electrochemical gas sensor will be described.
The sensor has the structure in which a synthetic resin holder (9), and oxygen permeable film pressing member (6), membrane pressing member (14) are used, and the gas permeable membrane (12), working electrode (11), counter electrode (8), reference electrode (13), electrolyte holding member (10), and lead wires (1), (2) and (4) to make electrical conduction to each electrode are arranged, and the electrolyte (7) is hermetically sealed inside. The electrode is the catalyst whose main component is the precious metals such as platinum or platinum black, and by which the gas having the reduction action such as carbon monoxide, hydrogen, or the like, or the gas having the oxidation action is effectively oxidized or reduced on the working electrode (11).
The potential of the working electrode (11) is held at a suitable value for the oxidation and reduction response of the gas to the reference electrode (13) by the external circuit shown in FIG. 2. In this case, the current is not circulated to and from the reference electrode which is the reference of the potential, and which only regulates the potential of the working electrode and has no relation to the reaction.
Further, because the potential of the counter electrode is not regulated, the potential of the counter electrode is the natural electrode potential for the reaction corresponding to the reaction on the working electrode. Accordingly, the oxidation and reduction reaction of the gas to be measured occurs only on the working electrode, and the reaction of the other side occurs only on the counter electrode.
When the gas to be measured is diffused from the outside in the permeable membrane (12) and reaches the working electrode (11), the oxidation reaction shown in the relational expression (A) occurs. On the one hand, simultaneously, on the counter electrode (8) through the electrolyte (7), the reduction reaction of the oxygen shown in the relational expression (B) occurs. The oxygen diffuses in the oxygen permeable membrane (5) from the atmosphere in which the sensor is used, and is dissolved in the electrolyte (7), and diffused in the electrolyte and reaches the counter electrode (8).
(Working electrode reaction) CO+H2Oxe2x86x92CO2+2H++2exe2x88x92xe2x80x83xe2x80x83(A)
(Counter electrode reaction) 1/2O2+2H++2exe2x88x92xe2x86x92H2Oxe2x80x83xe2x80x83(B)
(Total reaction) CO+1/2O2xe2x86x92CO2xe2x80x83xe2x80x83(C)
At this time, the current flows between the working electrode and the counter electrode is shown by the relational expression (D), and is proportional to the concentration of the gas to be measured; therefore, by leading the current through the lead wire (4) connected to the working electrode and the lead wire (2) connected to the counter electrode to the outside, the concentration of the gas to be measured can be detected.
(the relationship of the reaction current and gas concentration)                     i        =                                            F              xc3x97              A              xc3x97              D              xc3x97              C                        σ                    xc3x97          n                                    (        D        )            
Where:
i: reaction current
F: Faraday constant
A: area of the diffusion surface
D: diffusion coefficient of the gas
C: concentration of the gas
"sgr": thickness of the diffusion layer
n: number of reaction electrons
(in the reaction of the sensor, F, A, D, "sgr", n are constant)
In the case of the reaction, concerning the water (H2O) consumed on the working electrode by the oxidation of the carbon monoxide gas, because the amount of the equivalent is generated by the reduction of the oxygen (O2) in the outside air on the counter electrode, there is no chemically consumed component.
However, in the practical use, there is a case in which the sensor is not operated normally, by the extrinsic factors such as the aging deterioration of members constituting the sensor or the contact continuity condition, or the stain of the membrane through which the gas diffuses and penetrates.
Accordingly, when ordinarily, the poisoning alarm unit or the measuring device using such the electrochemical carbon monoxide gas sensor is used, the inspection or correction of the sensor output is necessary before using, and when the use is for a long period of time even in the continuous use, it is necessary that the measurement is periodically stopped for the correction or replacement of the sensor so that the accuracy or reliability is maintained. Conventionally, the correction of the sensor is conducted in such a manner that the maintenance man or the user himself flows the correction gas including a predetermined concentration carbon monoxide into the sensor and the sensor output generated at the time is measured. However, it is very troublesome to conduct such the correction operation periodically, and there is a possibility that the correction operator is exposed to the carbon monoxide gas for correction.
Further, due to such difficulties, the periodical inspection may not be conducted, and thus, in the case where the concentration of the carbon monoxide of the atmosphere is increased, and a possibility of poisoning occurs, the sensor may not normally operate, and the alarm may not be provided accordingly.
The present invention is described in the following (1) to (9).
(1) A testing method of an electrochemical gas sensor, in which: a working electrode which electrochemically oxidizes or reduces the first gas component to be detected, a counter electrode which acts electrochemical reduction reaction or oxidation reaction corresponding to an oxidized or reduced amount of the first gas component, and an electrolyte are provided; and an sensor output which is a value of the oxidation current or reduction current of the first gas component, is calculated and the concentration of the first gas component is detected, the testing method of an electrochemical gas sensor, which includes the steps of that: in which the voltage in which the current flows in the reverse direction to the oxidation current or reduction current of the first gas component, is applied between the working electrode and the counter electrode from the outside; and after the second gas component is generated on the working electrode so as to be a predetermined concentration by the electrolysis of the electrolyte, the sensor output which is a value of the oxidation current or reduction current on the working electrode of the second gas component, is measured, and in which the second gas component shows a sensor at output practically proportional to the sensor output of the first gas component in the concentration corresponding to the concentration of the first gas component.
(2) The testing method of an electrochemical gas sensor described in (1), in which the sensor is tested by using a correction value which is a ratio of the output of the measuring sensor in the predetermined concentration of the second gas component, and the sensor output calculated according to the known reference from the predetermined concentration of the second gas component.
(3) The testing method of an electrochemical gas sensor described in (1), in which the sensor is tested by using a correction value which is a ratio of the second gas component concentration calculated according to the known reference from the measuring sensor output, and the predetermined concentration of the second gas component.
(4) The testing method of an electrochemical gas sensor described in (1), in which the first gas component is carbon monoxide.
(5) The testing method of an electrochemical gas sensor described in (4), in which the electrolyte is an aqueous solution, and the second gas component is hydrogen.
(6) The testing method of an electrochemical gas sensor, in which the testing is correction, and/or deterioration judgement, and/or life judgement.
(7) An electrochemical gas sensor, in which a testing means according to the testing method described in (6), is provided.
(8) An apparatus which is provided with an electrochemical gas sensor, and a testing means according to the testing method described in (6).
(9) A control apparatus of the electrochemical gas sensor which is provided with a testing means according to the testing method described in (4).
As described above, at the time of the correction of an electrochemical carbon monoxide gas sensor, the present invention judges the deterioration of the sensor by means of temporarily operating the electrode potential by the outside, and generating the gas having the same action as at the time when the correction gas flows, on the electrode, and then, returning the potential to the normal potential, and by the reaction of the sensor upon the generated and remained gas, and can correct the sensor, without practically using the correction gas including carbon monoxide.